Opsis

A native macOS markdown viewer designed for reading.

Opsis treats Markdown files as documents to be read, not code to be reviewed. The rendering is typographically refined -- Charter serif body text on a warm cream background, editorial-quality blockquotes, ornamental section breaks, and OpenType features like ligatures and old-style numerals. It's closer to a printed book than a GitHub README.

The name comes from the Greek opsis (opsis), meaning "sight" or "view."

Design

Opsis follows an editorial/literary aesthetic. The goal is a reading experience where the interface disappears and the content feels effortless.

• Charter serif for body text -- warm, screen-optimized, bundled on macOS. San Francisco sans-serif for headings creates natural hierarchy.
• Warm color palette -- cream backgrounds, ink-like text, warm charcoal in dark mode. No pure white, no pure black.
• Typographic details -- hanging punctuation, ligatures, old-style numerals, decorative blockquote marks, gradient-fade section breaks with ornamental glyphs.
• Reading-optimized layout -- 720px content width for ~65-75 characters per line, generous line height (1.75), and spacing that serves the serif.

Features

• GitHub Flavored Markdown -- tables, task lists, strikethrough, autolinks, and more via cmark-gfm
• Syntax highlighting -- 20+ languages powered by highlight.js
• Dark mode -- follows system appearance automatically with warm dark palette
• Document-based -- double-click .md files or use File > Open
• Finder integration -- registers as a viewer for .md, .markdown, and .mdown files
• Smart typography -- curly quotes, en-dashes, and em-dashes
• External links -- links open in your default browser, not in-app

Requirements

• macOS 26 (Tahoe)
• Xcode 26.2+

Building

The Xcode project is generated from project.yml using xcodegen:

xcodegen generate
xcodebuild -scheme Opsis build

Testing

Three test layers cover the rendering pipeline end-to-end:

# Unit tests — MarkdownRenderer, HTMLTemplate, MarkdownDocument
xcodebuild -scheme Opsis -only-testing OpsisTests test

# Integration tests — full pipeline verified via WKWebView + evaluateJavaScript
xcodebuild -scheme Opsis -only-testing OpsisIntegrationTests test

# UI tests — app launch, document opening, rendered content visibility
xcodebuild -scheme Opsis -only-testing OpsisUITests test

# All tests
xcodebuild -scheme Opsis test

Tech Stack

• Swift 6.2 / SwiftUI with DocumentGroup
• cmark-gfm for GFM markdown parsing
• Native SwiftUI WebView (macOS 26) for rendering
• highlight.js 11.9.0 for syntax highlighting
• xcodegen for project generation